Activity to Teach the Inverse-Square Law for Radiation Flux

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Presentation transcript:

Activity to Teach the Inverse-Square Law for Radiation Flux Context: This activity is used in a 300-level course on Earth Systems and Environmental Science (using Kump, Kasting, and Crane text), but could be used in an intro class, as well. This is used at the beginning of a section on Energy Balance. We just covered electromagnetic radiation and now are beginning a section on radiation flux. Objectives: Students will be able to demonstrate how the inverse-square law for radiation flux works. Students will be able to define terms in the inverse-square law and apply this law to an example problem.

Radiation Flux What do I mean by the term “flux” ???? We cover what “radiation flux” is after covering the electromagnetic spectrum: Radiation Flux What do I mean by the term “flux” ???? rate of energy or material that passes through a given area over a period of time flux of water in a reservoir may be given in m3/day (volume per unit time)

Radiation Flux DEMO... Getting to the concept of flux per unit area: DEMO: shine light on paper. If paper is far away, is light as bright on it? Why. Tip paper. What now? Do we have a solar power toy to do demo with? figure 3-4 DEMO...

How much flux does Venus get versus Earth? Mars? Radiation Flux How much flux does Venus get versus Earth? Mars? To answer this, how might you go about estimating radiation flux to these different planets (assume you can measure flux on Earth, which we can!)? Which planet do you think will have greater radiation flux? Why? What parameters might be critical to understand from this concept? Can you write a functional equation that captures this concept [e.g., y = f(x)]?

Activity: With the flashlight, ruler, and a piece of paper, do the following: Shine the flashlight on your paper from 5cm distance (call this r0). Measure the diameter of the resulting spot of light. Do the same using the light source 10cm distance (call this r). Assume that the energy from the light is 100 units. Calculate how much ‘energy’ per unit area is received at the paper for each distance (call the value for 5cm distance, S0 and the value for 10cm distance S). Take the ratio of these numbers . Next, play with different ratios of your distance from the paper (r0 and r) and see if you get a number that is close to the ratio of energy.

Inverse-Square Law Rearrange this equation to put the different ratios on opposite sides. Is this close to your equation? NOTE: Energy from the Sun is released in a spherical pattern, so the geometry of the system is 3-dimensional while yours is 2-dimensional. Surface area of sphere = 4πr2 So = energy at reference

How much flux does Venus get versus Earth? Mars? Radiation Flux How much flux does Venus get versus Earth? Mars? Inverse-Square Law Distance from Sun: Venus: 0.72 AU Earth: 1.00 AU Mars: 1.52 AU Earth Flux: 1370 W/m2 What is flux on Venus and Mars? Surface area of sphere = 4πr2 So = energy at reference

How much flux does Venus get versus Earth? Mars? Radiation Flux How much flux does Venus get versus Earth? Mars? Distance from Sun: Venus: 0.72 AU Earth: 1.00 AU Mars: 1.52 AU Earth Flux: 1370 W/m2 What is flux on Venus and Mars? Venus: Mars: